The U.S. aluminum industry is one of the largest in the world with about 2.5 million metric tons of primary aluminum produced in 2005. Presently, the aluminum industry relies on three major processes for primary aluminum production: alumina refining from bauxite, anode production, and aluminum smelting by electrolysis in the Hall process. Hall electrolytic cells electrochemically reduce alumina to aluminum metal via carbon anodes and molten aluminum cathodes in the smelting process. Smelting is the most energy intensive step in primary aluminum production and accounts for between 2% and 3% of the electricity used in the U.S. every year (about 15 kWh/kg aluminum produced). Smelting also results in a variety of emissions, effluents, by-products and solid wastes. Greenhouse gases are a major pollutant from aluminum production and are caused by fossil fuel consumption, carbon anode consumption, and perfluorocarbons from anode effects. Emissions from anode production include particulates, fluorides, polycyclic aromatic hydrocarbons (PAH) and sulfur dioxide (SO2). Emissions from aluminum smelting include carbon monoxide (CO), carbon dioxide (CO2), SO2, fluorides, perfluorocarbons (PFCs, e.g., CF4, C2F6), and PAH. It would be advantageous to lower costs and reduce waste to remain competitive with foreign producers. The smelting step is a priority area for improvement because of high energy use and undesirable emissions and by-products implicated in climate change.
Carbothermic reduction of aluminum is an alternative process for aluminum production. Carbothermic aluminum production involves using carbon and temperature changes to effect production of aluminum. Carbothermic processes require much less physical space than the Hall electrolytic reduction process and could result in decreased electrical consumption. Long term estimates suggest the carbothermic process could reduce energy requirement by over 30% to about 8.5 kWh/kg. Carbothermic production of aluminum would also eliminate perfluorocarbon emissions resulting from carbon anode effects, hazardous spent potliners, and hydrocarbon emissions associated with baking of consumable carbon anodes. Thus, carbothermic production of aluminum would be more energy efficient and have less environmental impact than traditional aluminum production processes.
The direct carbothermic reduction of alumina to aluminum has been described in U.S. Pat. No. 2,974,032 (Grunert et al.), U.S. Pat. No. 4,099,959 (Dewing et al.), U.S. Pat. Nos. 4,033,757; 4,334,917; 4,388,107; and 4,533,386 (all Kibby), U.S. Pat. No. 6,440,193 (Johansen and Aune), U.S. Patent Publication No. US2006/0042413 (Fruehan), the Proceedings 6th Conference on Molten Slags, Fluxes and Salts, Edited by S. Seetharaman and D. Sichen “Carbothermic Aluminum”, K. Johansen, J. Aune, M. Bruno and A. Schei, Stockholm, Sweden-Helsinki Finland, Jun. 12-17, 2002, and “Aluminum Carbothermic Technology Alcoa-Elkem Advanced Reactor Process”, Light Metals 2003, 401-406.
The overall aluminum carbothermic reduction reaction:Al2O3+3C→2Al+3CO   (1)
takes place, or can be made to take place, generally in steps such as:2Al2O3+9C→Al4C3+6CO (vapor)   (2)Al4C3+Al2O3→6A1+3CO (vapor)   (3)Al2O3+2C→Al2O (vapor)+2CO (vapor)   (4)Al2O3+4Al→3Al2O (vapor)   (5), andAl→Al (vapor)   (6).
A large quantity of aluminum vapor species may be formed during various ones of the above reactions. To recover such vapor species, and the latent and sensible heat they contain, an external vapor recovery unit or vapor recovery reactor (VRR) may be employed. In the VRR, gases containing Al2O and Al vapors react with carbon to produce Al4C3 or Al4C3—Al2O slag. Examples of reactions that may occur in the VRR are provided below:2 Al2O(g)+5C→Al4C3+2CO   (7)5C≠C3+C4 Al(g)+3C→Al4C3   (8)
Prior methods of recovering Al vapor and Al2O from off-gases generated during carbothermic reduction of alumina are disclosed in U.S. Pat. No. 6,530,970 (Lindstad), U.S. Pat. No. 6,849,101 (Fruehan), and Fruehan et al., “Mechanism and Rate of Reaction of Al2O, Al, and CO Vapors with Carbon”, Metallurgical and Materials Transactions B., 35B, 617-623 (2004). Such references generally propose the use of hydrocarbons or charcoal for reaction with the off-gases. Furthermore, liquid hydrocarbon product may cause bridging of the particles in the reactor making it difficult to operate the vapor recovery reactor. Solid carbon particles may also become covered by reaction products, thereby reducing the reaction rate, eventually resulting in unreacted carbon entering the main carbothermic furnace, which is undesirable. Charcoal has good surface area and conversion rates, but is generally four times as expensive as petroleum products.